HOLLYWOOD’S major studios have totally avoided releasing movies with NC-17 ratings since the notorious “Showgirls” 10 years ago, but that will change Friday with Universal’s release of the documentary “Inside Deep Throat.”

“We wanted to show the full-on fellatio,” Randy Barbato, who co-directed with Fenton Bailey, said of explicit footage from the 1973 porn classic – whose legal and sociological impact is examined in the documentary.

“To take that scene out would have been coy. It was a really political moment.”

For a decade, the NC-17 rating was considered the box-office kiss of death, because of the assumption that many theaters wouldn’t book films with the rating – which bars anyone under 17, accompanied or not – and many newspapers wouldn’t carry ads.

None of these did particularly well, but SPC’s release of Pedro Almodovar’s “Bad Education,” which carried the same rating because of its sex scenes, has taken in a quite respectable $3.7 million so far in limited release.

“Inside Deep Throat” is going out via Universal – rather than the studio’s specialty division, Focus Features – because it comes from Universal’s top producer, Brian Grazer, and his Imagine Entertainment (Ron Howard is his partner), which turned out such hits as “A Beautiful Mind” and “8 Mile.”

Grazer admits there was a little nervousness when Universal was acquired by General Electric’s NBC division during the documentary’s production.

“But Universal and GE are totally behind the project,” Grazer told The Post before the film’s premiere recently at the Sundance Film Festival. “It’s a First Amendment issue for them.”

* Among the many other notable documentaries at Sundance, one that stands out is veteran publicist Dan Klores’ “Ring of Fire,” a compelling look at one of New York’s biggest stories of 1962, the death of boxer Benny “Kid” Paret after a welterweight match at Madison Square Garden with champ Emile Griffith.

I also liked Marc Levin’s “The Protocols of Zion,” a trenchant look at anti-Semitism; and Kirby Dick’s “Twist of Fate,” about a devout Catholic in Toledo, Ohio, whose life crumbles when he confronts a priest who allegedly abused him as a child.

* The work of James Cagney has long been criminally neglected on DVD, but Warner Home Video has made amends by including four of the feisty actor’s best films in a new collection of gangster classics.

Cagney’s criminal characters meet horrible ends in all four movies, shot to death and dumped on his mother’s doorstep in 1931’s “The Public Enemy” and fried in the electric chair as he demonstrates cowardice in 1938’s “Angels With Dirty Faces.”

Even more unforgettable are Cagney’s balletic death sprawl on the steps of a church after being shot by Humphrey Bogart in the nostalgic “The Roaring Twenties” (1939) – and, best of all, being incinerated atop a flaming gasoline tank at the climax of “White Heat” (1949) as he proclaims “Made it, Ma! Top of the world!”

For a pistol-packing female star, check out the opening of another new Warner release, Bette Davis in William Wyler’s adultery classic “The Letter.” This one includes a fascinating alternative ending without Davis’ famous curtain line, “I still love the man I killed!”